A printing method by an electrostatic transfer with a toner includes an electrophotographic system such as an analog copying machine or a digital printer. Recording papers used for these printing apparatus generally include papers including recording papers for an over head projector (OHP), tuck sheets, etc. Label sheets are mainly used as label papers such as papers for bar code, etc., posters, etc.
A general layer structure of the label sheet is designed to form a resin layer on a film at a label side in order to increase the adhesion to a toner and also to obtain a more stable running property of a sheet by increasing the electrical conductivity of the surface to reduce electrostatic charging by static electricity.
Coated papers are mainly used as base films of these label papers and separate papers. In bar code label paper for managing an article, etc., when the article is changed, it is necessary to change the bar code. In this case, in the step of stripping off the sheet, there is a possibility to break the bar code itself and hence such a conventional bar code label paper is not practical. Further, in a high humidity district, a coated paper is largely deformed with moisture and hence in such a case a synthetic paper is preferably used as the base film. As the base material for such a synthetic paper, a polypropylene resin is mainly used alone or as a laminated layer thereof. The synthetic paper has a less possibility to be broken by the re-peeling thereof but severe curling occurs by the heat (140.degree. to 150.degree. C.) of a fixing roll in the case of using the synthesis paper.
However, recently, the printing speed is more and more increased and a higher fixing temperature is required, whereby the use of a thermally stable heat-resistant film has been investigated. In the label paper using such a heat-resistant base film, deformation and curling do not occur at an ordinary fixing roll temperature (130.degree. to 150.degree. C.) but in the case of using a fixing roll at higher temperature (e.g., 200.degree. C.), the heat shrinking extent differs between the label side and the separate side of the label paper to cause severe curling and deformation.
To overcome the problem, it is proposed to use a same synthetic resin film as the films at the label side and at the separate side (see, JP-UA-63-24299; the term "JP-U-A" used herein means an "unexamined published Japanese utility model application"). However, even when such a method is used, by using a synthetic resin film which is unstable in a stretching extent of the synthetic resin film and is thermally unstable, curling occurs. Further, if a synthetic resin film wherein the thermal deformation differs between both the surfaces thereof is used, large curling occurs.